Re: Statically propagate basic blocks which are likely executed 0 times

[Renlin Li]

Hi Honza & Christophe,

I have tested your suggested fix. It does fix the regression.
Here is a simple patch for it.

After r249013, die () and dump_stack () are both in cold section. This makes
the compiler generate bl instruction for the function call, instead of
honoring the -mlong-calls option.

This patch changes the dump_stack function call conditional, which fixes the
regression.

Okay to commit?

Regards,
Renlin

gcc/testsuite/ChangeLog:

2017-06-12  Renlin Li  <renlin...@arm.com>

        * gcc.target/arm/cold-lc.c: Update coding style, call dump_stack
        conditionally.


On 09/06/17 10:54, Jan Hubicka wrote:

Since this commit (r249013), I've noticed a regression on arm targets:
FAIL: gcc.target/arm/cold-lc.c scan-assembler-not bl[^\n]*dump_stack

I think that is because we optimize the testcase:
/* { dg-do compile } */
/* { dg-options "-O2 -mlong-calls" } */
/* { dg-final { scan-assembler-not "bl\[^\n\]*dump_stack" } } */

extern void dump_stack (void) __attribute__ ((__cold__)) __attribute__ 
((noinline));
struct thread_info {
     struct task_struct *task;
};
extern struct thread_info *current_thread_info (void);
extern int show_stack (struct task_struct *, unsigned long *);

void dump_stack (void)
{
     unsigned long stack;
     show_stack ((current_thread_info ()->task), &stack);
}

void die (char *str, void *fp, int nr)
{
     dump_stack ();
     while (1);
}

the new logic will move die() into cold section (because it unavoidably leads 
to cold
code and thus allow using the bl instruction.
I guess just modifying die to call dump_stack conditionally should fix the 
testcase.

Honza

+  if (!n->analyzed
+      || n->decl == current_function_decl)
+    return false;
+  return n->frequency == NODE_FREQUENCY_UNLIKELY_EXECUTED;
+}

diff --git a/gcc/testsuite/gcc.target/arm/cold-lc.c b/gcc/testsuite/gcc.target/arm/cold-lc.c
index 467a696..f0cd6df 100644
--- a/gcc/testsuite/gcc.target/arm/cold-lc.c
+++ b/gcc/testsuite/gcc.target/arm/cold-lc.c
@@ -11,13 +11,14 @@ extern int show_stack (struct task_struct *, unsigned long *);
 
 void dump_stack (void)
 {
-    unsigned long stack;
-    show_stack ((current_thread_info ()->task), &stack);
+  unsigned long stack;
+  show_stack ((current_thread_info ()->task), &stack);
 }
 
 void die (char *str, void *fp, int nr)
 {
+  if (nr)
     dump_stack ();
-    while (1);
+  while (1);
 }